With the size and resolution of LCD displays continuously increasing, projection photolithography tools, one of the most essential processing equipment in the field of LCD displays, are faced with a number of technical challenges. Nowadays, in order to meet the demand for larger-sized LCD displays with higher resolutions, most photolithography tools of the latest generations employ either of the following two approaches to obtain a high exposure yield of large-sized glass substrates: 1) use of a single large-sized projection objective lens, adopted for example by the Japanese company Canon Inc. as a solution for latest-generation exposure photolithography; and 2) use of a combination of multiple objective lenses, adopted for example by the Japanese company Nikon Corporation and details of which can be seen with reference to the patent document CN1459671A entitled “Exposure Method, Exposure Apparatus and Device Fabrication Method”.
With reference to a schematic illustration of an exposure apparatus according to CN1459671A, the exposure apparatus comprises, disposed sequentially in a vertical direction, an illumination optical system IL, a mask stage MST, a projection optical system PL and a photosensitive substrate P. The mask stage MST is configured to carry a mask M. During operation of the exposure apparatus, the mask M and the photosensitive substrate P move in synchronization relative to the projection optical system PL along the X axis, such that an area of the photosensitive substrate P within an exposure area projected by the projection optical system PL on the photosensitive substrate P is exposed. Before the exposure, alignment and focusing measurements must be performed in order to ensure pattern accuracy and overlay accuracy. However, defocus and tilt errors often occur during the alignment and focusing measurements, which may lead to significant degradation of alignment accuracy and hence have adverse impact on the yield rate of the electronic devices being fabricated.
Technicians in this art have been seeking for a solution for this problem.